1. An earphone; such as, those used in sound-powered phones, which has a diaphragm of dielectrically (very poor conductor of electric current) charged material including polarized plastic, thereby eliminating the need for an external power source.
2. A headphone consisting of an electret transducer, usually in the form of a push-pull transducer or an instrument; such as, a microphone or electric motor, which converts one form of energy into another form.